perf(ci): gate coverage collection to merge_group only — follow-up to #363

[scottschreckengaust]

Parent: #363. Spun from #363's DoD by the orchestrator; needs maintainer approved (ADR-003).

Goal

Skip collectCoverage on the high-frequency pull_request event; keep coverageThreshold/fail_under enforced on merge_group so nothing merges under-covered.

Scope

• Conditional coverage on event name (PR = off, merge_group = on).
• Verify the required build context still reports on PRs without coverage.

Refs #363